The present invention relates to an optical fiber collimator using a gradient index rod lens.
FIG. 1 shows a conventional collimator optical device 50 having incident side and receiving side optical fiber collimators. The incident side optical fiber collimator includes an optical fiber 11 and a rod lens L1, and the receiving side optical fiber collimator includes an optical fiber 12 and a rod lens L2. The optical device 50 converts lights emitted from the single mode fiber 11 on the incident side into collimated lights by use of the collimator lens L1, and condenses the collimated lights by use of the collimator lens L2 to couple them to the single mode fiber 12 on the receiving side. The collimator lenses L1 and L2 are gradient index rod lenses having a refractive index distribution in a radial direction.
Various kinds of collimator optical devices (devices for optical communications) 50 are produced by inserting an optical function element (e.g., an optical filter, an optical isolator, an optical switch or an optical modulator) between the rod lenses L1 and L2. The device for optical communications causes a predetermined function to a light having propagated through the optical fiber 11 by use of the optical function element, and then couples the light again to the optical fiber 12. In order to use a function element (e.g., a large-sized matrix switch) requiring a long light path length and having a large size to cause the predetermined function, it is required to provide a device for optical communications having as great opposing distance (maximum collimation length Lmax) between the rod lenses L1 and L2 as possible, and as high coupling efficiency as possible.
FIG. 2 shows an optical fiber collimator 10 used in the collimator optical device 50. The optical fiber collimator 10 includes a gradient index rod lens 13, a single mode fiber 14, a capillary 15 for holding the optical fiber 14, and a glass tube 16. An incident side end face of the rod lens 13 and an end face of the optical fiber 14 are each inclined planes obliquely buffed. The rod lens 13 and the capillary 15 are fixed inside the glass tube 16 at a position where the incident side end face of the rod lens 13 and the end face of the optical fiber 14 are away from each other by a focal length of the rod lens 13.
In the optical fiber collimator 10, it is necessary to increase the focal length of the rod lens 13 and enlarge a beam diameter, in order to increase the opposing distance. The focal length of the rod lens 13 can be changed by adjusting a lens length Z of the rod lens 13. Here, the “lens length” is the length between both the end faces of the rod lens. In the case of the rod lens 13 having an inclined plane, the “lens length” is the distance from an intersection point of the inclined plane and a center axis to the incident side end face (see FIG. 6). Since the gradient index rod lens has a meandering period (pitch) of a ray determined by its refractive index distribution, the lens length Z is expressed by pitch as a unit.
For example, in the case of a normal rod lens having a lens element diameter of φ 1.8 mm and a lens length Z of 0.25 pitches, the opposing distance is about 70 mm. On the contrary, if the lens length is changed to 0.1 pitches, the opposing distance extends up to about 200 mm. If the lens length Z of the rod lens having a lens element diameter of φ 0.1 mm is changed from 0.25 pitches to 0.1 pitches, the opposing distance extends from about 20 mm to about 70 mm.
In the conventional optical fiber collimator 10, it is necessary to decrease the lens length Z in order to increase the opposing distance. For example, if the lens element diameter of the rod lens 13 is φ 1.8 mm and the lens length Z thereof is 0.23 pitches, the actual lens length Z is 4.8 mm. If the lens element diameter of the lens 13 is φ 1.8 mm and the lens length Z thereof is 0.1 pitches, the actual lens length Z is about 2 mm. If the lens element diameter of the lens 13 is φ 1.0 mm and the lens length Z thereof is 0.1 pitches, the actual lens length Z is 1.2 mm. However, if the lens length Z is small, the following problems are caused.
(1) As shown in FIG. 3, if a short rod lens 13A having a length of, for example, 1.2 mm is set to the glass tube 16, the rod lens 13A might incline because an axial length of an outer circumferential surface (referential surface) of the rod lens 13A is small. If the rod lens 13A inclines, the collimated light (emitted light) emitted from the rod lens 13A inclines with respect to the axial direction, which decreases the coupling efficiency. As a result, reliability might be decreased.
(2) If the length of the lens is small, it is difficult to cut or buff the lens when the rod lens 13A is manufactured. Especially, it is sometimes impossible to obliquely buff the end face of the lens. This is because it is difficult to hold the rod lens 13A in the cutting and buffing processing.
(3) It is difficult to handle the lens if the length of the lens is small.